Mild winter with few fatal avalanche accidents

The main reason for the little snow, even at high altitudes, was the low precipitation between November 2024 and April 2025 in most of the Swiss Alps. In contrast to the automatic IMIS stations located above 2000 m, the lack of snow in the winter of 2024/25 was less exceptional at the stations located below 1500 m, as mild winters at this altitude had already led to below-average snowfall in previous years.

The low snowfall at the start of winter meant that the snowpack had a weak foundation in many regions. This critical old snow situation with angular, soft layers deep in the snowpack persisted throughout the winter, particularly on shady slopes, especially in the snow-poor areas of southern Valais, Ticino and Graubünden. Frequent breaks in the weak old snowpack often led to large-scale avalanches and many medium and large avalanches. In contrast, the long-lasting weak layers in the snow-rich areas of the northern slopes of the Alps and the western lower Valais were usually covered by thick layers of well-consolidated snow.

The two most avalanche-active periods of the winter of 2024/25 were in the week at the end of 2024 and at the end of January 2025. 10 people died in 9 avalanche accidents up to mid-April 2025. The number of fatalities is significantly lower than the average for the last 20 years, although the number of personal avalanches reported to the SLF is slightly above average.

The real onset of winter was a long time coming

Two onsets of winter were recorded in the mild fall of 2024. In the glaciated high mountains above 3000 m, winter already set in at the beginning of October. Here it snowed repeatedly during the rainy and cloudy October. At lower altitudes, however, most of the precipitation fell as rain. In contrast to October, November got off to a very sunny and exceptionally mild start in the mountains. As a result, by mid-November there was only snow cover on northern slopes above 2,800 m and in the high mountains.

It only became really wintry in the second third of November: an extraordinary amount of snow fell in the west and north from November 19 to 22, 2024, with gale-force winds in some places, down to low altitudes. The extensive accumulations of drift snow were deposited on an unfavorable, angular snow surface, especially on shady slopes. The avalanche danger increased significantly, at times to high (level 4) in the high mountains. Avalanches were mainly registered by automatic detection systems and reported from high-altitude ski resorts. After each autumn snowfall, many small sliding snow avalanches occurred on the still warm ground at medium and high altitudes.

White Christmas - lots of fresh snow and an "old snow problem" in the mountains

It snowed repeatedly in December 2024. There was a great Christmas present from December 21 to 24, 2024: in the west and north, persistent and intensive snowfall provided plenty of fresh snow in the Swiss Alps, which improved the snow conditions above 2000 m there. Along the northern Alpine ridge from Lower Valais to the Glarus Alps and in the Gotthard region, 1 to 1.5 m of snow fell across the board. The remaining areas in the north received around half a meter of snow. The south and the Engadin, on the other hand, came away empty-handed.

Even before this major snowfall, it was clear that the entire old snow cover was a weak foundation for the winter. Several high-pressure, dry periods with long, clear nights meant that the snow had built up. In many places, a pronounced weak layer of large, angular crystals with little bonding to each other formed, which later resulted in an old snow problem close to the ground.

The large quantities of new and drift snow before Christmas covered the weak foundation - and the combination of layers necessary for slab avalanches was complete. On December 23 and 24, 2024, there was a critical avalanche situation in the west and north and widespread high avalanche danger (level 4). Fractures in the weak old snow led to large-scale avalanche starts and many large and very large spontaneous avalanches.

Between Christmas and New Year's Eve, it was sunny, windy and exceptionally mild in the mountains for this time of year, making for the best touring and freeriding conditions. However, it was one of the most avalanche-active periods of winter 2024/25: in just 7 days, 98 avalanches were triggered by snow sports enthusiasts, with 22 people caught and three completely buried. The size of the avalanches was impressive: 58 medium and 27 large avalanches were reported. The large-scale spread of avalanches can be explained by the pronounced and very widespread weak old snow cover. The avalanche danger therefore remained considerable in many areas of the Swiss Alps (level 3). In addition, the Christmas snowfall and the subsequent mild temperatures led to many sliding snow avalanches, especially in the snow-rich areas on the northern slopes of the Alps.

Stormy start to the year - the wind as the master builder of avalanches

In addition to the long-lasting old snow problem, the focus in winter 2024/25 was also repeatedly on short-lived drift snow problems: stormy conditions repeatedly led to instabilities in the new and drift snow layers or at the transition to the old snow near the surface, as on January 18, 2025. In the foehn regions of the north, strong southerly föhn winds transported loose old snow near the surface onto an unfavorable old snow surface, which thus became a weak layer. As a result, many avalanches were triggered by people in thin, platy accumulations of drift snow. By January 19, 2025, the situation had already calmed down, which is typical for a drift snow problem.

Plenty of snow in the south - Ticino as the snowfall center of Switzerland

While the first and last third of January was changeable in the north and very mild with southerly foehn winds, the south and west saw plenty of snow in several batches. A southern dust situation followed by a pronounced cold front brought particularly heavy snow between 25 and 29 January 2025. More than 1 m of snow fell in the west and on the main Alpine ridge from the Lukmanier to the Bernina Pass and south of it, otherwise around half a meter fell on both sides of the Alps. The large amount of new and drifting snow was deposited on the thin and extremely weak snow cover. The high avalanche danger forecast for January 28, 2025 (level 4) was confirmed by the highest avalanche activity of winter 2024/25 with many large and occasionally very large spontaneous avalanches. On January 29, 2025, the first day of good weather after the heavy snowfall, the situation for winter sports away from the secured slopes was very critical. On this day, winter sports enthusiasts triggered many medium and large avalanches.

From February to the beginning of March, it was mild, dry across the board and there was plenty of sunshine in the mountains and in the south. After the heavy snowfall at the end of January, the old snow problem quickly calmed down. There were only isolated avalanche reports with triggering in the deep layers of the snowpack and large-scale avalanches. As a result, the avalanche situation was mostly favorable from February to the beginning of March.

From March 9 to 17, 2025, a striking southerly dust situation caused a wintry interlude with plenty of snow in the south. Within a week, up to 1.5 m of snow fell in Ticino and the Simplon region above around 1800 m, but up to 1 m of snow also fell in the west on the Great St. Bernard and in the east from Bergell to the Bernina region. The snow cover regained momentum: in the south, the snow had once again fallen on an unfavorable old snow surface consisting of loose layers with an angular structure, especially on northern slopes. As a result, the new and drifting snow layers were prone to disruption and avalanches could be triggered in the upper part of the snowpack. In addition, in Valais, Ticino and Graubünden, weak layers deep in the snowpack became susceptible to disruption again. Here, avalanches started again in the old snow close to the ground, especially on shady slopes. On southern slopes, on the other hand, the situation was more favorable, as the snowpack here was moist up to high altitude before the snowfall. The large amounts of fresh snow caused the avalanche danger to rise to level 4 (large) at times, especially in the areas with a weak old snowpack in the south and in southern Graubünden. Avalanche activity was high here with numerous spontaneous and human-triggered avalanches. In the Münstertal in particular, the snowpack structure was exceptionally weak, making it a reliable area for breaks in the old snow close to the ground: dangerously large spontaneous avalanches and avalanches triggered by people in the old snow were reported here throughout the winter.

Final winter spurt - fresh snow, wind and many avalanches triggered by people

Who would have thought it - winter caught up again from 29 March to 1 April 2025, even in the snow-poor north-east, with up to half a meter of snow. However, fresh snow and loose old snow were transported by the strong north wind and deposited on shady slopes on an unfavorable old snow surface. The result: several days with many spontaneous and person-triggered avalanches. The drift snow problem was compounded by an old acquaintance, the weak old snow close to the ground. While the weak layers in the old snow in Ticino were now heavily overlaid, making it difficult to initiate breaks, Graubünden remained the "old snow hotspot" of the Swiss Alps with several avalanches triggered even in deep layers of the snowpack.

And what about wet avalanches? From mid-March to mid-April, the soaking of the snowpack with warmth and sun made significant progress: on southern slopes up to the high mountains, on eastern and western slopes up to around 2600 m and on northern slopes up to around 2000 m. The marked influence of warmth on the snowpack caused wet and sliding snow avalanches, but the avalanche periods remained less intense until mid-April.

Source: slf.ch